5G New Radio (NR) | Teleinf Edu

1. 5G NR requirements beyond the LTE air interface

The 4G LTE air interface was a major platform for mobile services in its time. It used OFDMA on the downlink and SC-FDMA on the uplink, providing efficient mobile broadband. As the next generation took shape, its fixed radio structure no longer matched the diverse requirements of emerging services. Mobile communication now included devices, sensors, and critical applications alongside faster smartphones.

The LTE air interface followed a "one-size-fits-all" design. Key signal parameters were fixed and optimized primarily for mobile broadband. For example, the was $15$ kHz, while frame and slot structures remained largely inflexible. This design handled fast phone data well, yet it did not cover every use case developed with 5G:

Ultra-Reliable Low-Latency Communications (URLLC):Mission-critical applications such as autonomous vehicles or remote surgery consider latency targets of a few milliseconds. LTE's fixed timing structure limited such short transmissions and flexible scheduling.
Massive Machine-Type Communications (mMTC): The IoT demands connectivity for billions of simple, low-power sensors. A representative sensor may send only a few bytes per week and operate for about a decade on a battery, so the full LTE radio capability could be energetically expensive for that profile.
Diverse Spectrum Bands: Mobile broadband (eMBB) also required new, wide spectrum blocks, including millimeter-wave ranges. NR specifications describe these ranges as FR1 and FR2; representative device ranges are FR1 from $410$ MHz to $7125$ MHz, FR2-1 from $24250$ to $52600$ MHz, and FR2-2 from $52600$ to $71000$ MHz. LTE parameters designed mainly for lower bands did not cover every property of mmWave signals.

A more flexible air interface was needed, one that could be configured for different services and frequency bands. This led to 5G New Radio (NR), which 3GPP continues to evolve through successive releases.

2. 5G NR configurability

5G NR separates the air-interface configuration into radio parameters, numerologies, slot formats, and bandwidth choices. 3GPP defines these elements as capabilities selected for a band, service, and deployment. A particular device and network support only a subset of those parameters, determined by the release and radio profile. Conformance specifications further constrain the supported bands, bandwidths, and numerologies for each device. The selected numerology also constrains symbol duration, slot timing, cyclic-prefix overhead, and sensitivity to phase noise.

The configuration has four practical aspects:

Frequency Ranges: 3GPP defines NR FR1 from $410$ MHz to $7125$ MHz, FR2-1 from $24250$ to $52600$ MHz, and FR2-2 from $52600$ to $71000$ MHz. Specific bands and device support depend on the release and radio profile.
Flexible Waveform Numerology: NR uses a flexible set of parameters, called numerology, instead of one fixed radio configuration. These parameters can be selected for latency, reliability, or spectral efficiency.
Unified Air Interface for All Services: 5G NR provides a single, unified radio framework that can efficiently support all three core 5G use cases (eMBB, URLLC, and mMTC).
Release Evolution: Later releases can add features, frequency bands, and service types. Compatibility between a device and a network depends on their common subset of supported releases, bands, and procedures.

3. 5G NR numerology and subcarrier spacing

One of the central concepts in the 5G NR air interface is its flexible numerology. In a radio waveform, numerology means the parameters that define its structure in time and frequency. LTE used one basic numerology, while 5G NR defines a family of scalable numerologies.

What is Numerology?

The 5G NR waveform is based on . CP-OFDM is used on the downlink, while the uplink can use CP-OFDM or DFT-s-OFDM depending on the configuration. Numerology is described by a specific Subcarrier Spacing (SCS), which changes the duration of other waveform parameters in the opposite direction:

Symbol Duration: The period of time used to transmit a single OFDM symbol. If you double the subcarrier spacing, the symbol duration is halved.
Cyclic Prefix (CP) Duration: A small copy of the end of an OFDM symbol that is prepended to the beginning. The CP acts as a guard interval to prevent inter-symbol interference caused by multipath delays. Its duration also scales inversely with the SCS.

The Scalable Numerologies of 5G NR

5G NR defines a set of numerologies based on scaling the foundational 15 kHz subcarrier spacing of LTE by powers of two. The standard specifies a parameter, $\mu$ (mu), to define the SCS:

\text{SCS} = 15 \times 2^\mu \text{ kHz}

This results in the following primary numerologies:

Numerology ( $\mu$ )	Subcarrier Spacing (SCS)	Typical Use Case	Characteristics
$0$	$15$ kHz	eMBB in low/mid bands, Coexistence with LTE	Long symbol duration, high spectral efficiency.
$1$	$30$ kHz	eMBB in mid-bands, some URLLC	Good balance between efficiency and latency.
$2$	$60$ kHz	URLLC, mmWave (FR2)	Short symbols support low latency and tolerance to phase noise.
$3$	$120$ kHz	URLLC, mmWave (FR2)	Very short symbol duration, lowest latency.
$4$	$240$ kHz	Example specialized services, depending on configuration	Short symbols for specific configurations.

4. The Trade-offs: Matching Numerology to the Service

Choosing a numerology involves trade-offs between latency, efficiency, and channel robustness. The configuration can therefore be matched to a particular service.

Narrow Subcarrier Spacing (e.g., 15 kHz)

This numerology results in a relatively long OFDM symbol duration.

Advantage - Spectral Efficiency: The cyclic prefix (CP) occupies a smaller proportion of a long symbol. Under comparable channel conditions, this can reduce overhead and improve spectral efficiency.
Advantage - Robustness to Multipath: The long symbol duration provides tolerance to delays caused by multipath propagation, similar to LTE.
Disadvantage - Higher Latency: A longer symbol extends transmission of a full slot. A longer increases waiting time, so the network can use a shorter time interval, a mini-slot, or another transmission format for URLLC requirements.
Typical Fit: eMBB services in lower and mid-frequency bands, when throughput and efficiency are important.

Wide Subcarrier Spacing (e.g., 120 kHz)

This numerology results in a much shorter OFDM symbol duration.

Advantage - Lower Latency: Shorter symbols mean a shorter slot. With 120 kHz SCS, the time assigned to one scheduling decision is shorter, supporting low-latency URLLC configurations.
Advantage - Resistance to Phase Noise: Very high frequency signals are more susceptible to phase noise. Wider subcarrier spacing increases the receiver's relative resistance to this effect.
Possible Cost - Overhead: The cyclic prefix occupies a larger proportion of a short symbol, so overhead can limit efficiency in a comparable configuration.
Disadvantage - Sensitivity to Multipath: The short symbol duration makes the system more sensitive to inter-symbol interference in environments with long delay spreads.
Typical Fit: URLLC configurations and millimeter-wave (FR2) transmissions when the shorter symbol justifies the additional overhead.

5. The 5G NR Frame Structure: Time and Frequency Resources

The 5G NR organizes its radio resources into a flexible grid of time and frequency, providing a structure for scheduling data.

The Time Domain: Frames, Subframes, and Slots

The basic timing structure is built around a $10$ ms radio frame, just like in LTE. However, how that frame is subdivided is much more flexible.

Radio Frame: The largest time unit, fixed at $10$ ms long.
Subframe: Each frame is divided into 10 subframes of $1$ ms each.
Slot: Here is where the flexibility of numerology comes in. A subframe is composed of one or more slots. The number of slots per subframe depends on the subcarrier spacing used:
- With 15 kHz SCS ( $\mu=0$ ), there is 1 slot per subframe.
- With 30 kHz SCS ( $\mu=1$ ), there are 2 slots per subframe.
- With 60 kHz SCS ( $\mu=2$ ), there are 4 slots per subframe.
- With 120 kHz SCS ( $\mu=3$ ), there are 8 slots per subframe.
OFDM Symbols: In the most common configuration, each slot consists of 14 OFDM symbols. The actual time duration of these 14 symbols becomes shorter as the SCS increases.
Mini-Slots: To support low latency for URLLC, 5G NR can schedule a transmission on fewer symbols than a full slot. Example formats include 2, 4, or 7 OFDM symbols, so the network can start a transmission before a full 14-symbol slot is available.

6. The Frequency Domain: Resource Blocks and Bandwidth Parts (BWP)

The frequency dimension of the radio grid is configurable as well.

Resource Block (RB): The smallest unit of frequency resources that can be allocated to a user is a Resource Block. An RB consists of 12 contiguous subcarriers. For a given numerology, all 12 subcarriers in an RB have the same spacing. The scheduler in the gNB allocates data to users in units of RBs and slots.
Bandwidth Part (BWP): A BWP is a subset of the Resource Blocks configured for a device. Compared with LTE, limiting the actively processed bandwidth can reduce power consumption when the active BWP is narrower than the carrier; exact behavior depends on the device and network configuration. The network can configure one or more BWPs, and the device monitors its active BWP instead of the full carrier bandwidth. For example, on a $100$ MHz carrier, a low-activity device might use a $20$ MHz BWP. When more capacity is needed, the network can switch it to a wider BWP, up to $100$ MHz in this example. This mechanism can save battery power and support devices with a narrower receiver bandwidth. For a serving cell, several BWPs may be configured while one is active for a given direction at a time, so the example describes a scheduling choice rather than an automatic energy guarantee.